Alpha-olefin oligomers and their use as synthetic lubricants are well-known. The oligomers are usually hydrogenated in order to improve their stability. Early reports of such oligomeric synthetic lubricants appear in Seger et al. U.S. Pat. Nos. 2,500,161 and Garwood 2,500,163.
Oligomerization of alpha-olefins in a Group IV metal oxide bed using a BF.sub.3 -protic promoter catalyst is described in U.S. Pat. No. 2,766,312. Promoters referred to therein include water, carboxylic acid, alkyl halides, alcohols and ethers.
U.S. Pat. No. 2,806,072 discloses the dimerization of C.sub.6 -C.sub.12 polypropylenes using a preformed BF.sub.3 -dialkyl ether catalyst.
Oligomerization of olefins using BF.sub.3 -promoter catalyst complexes of acid anhydrides, esters, ketones and aldehydes is described in U.S. Pat. No. 3,382,291.
U.S. Pat. No. 3,769,363 to Brennan discloses oligomerization of C.sub.6 -C.sub.12 normal alpha-olefins, such as 1-decene, with BF.sub.3 and C.sub.5 carboxylic acid to improve trimer yields.
U.S. Pat. No. 3,997,621 also to Brennan describes oligomerization of C.sub.6 -C.sub.12 normal alpha-olefins with BF.sub.3 using alcohols or water promoters in conjunction with small amounts of methyl and ethyl esters of a C.sub.2 -C.sub.5 monocarboxylic acid to improve trimer yields.
In U.S. Pat. No. 4,172,855 BF.sub.3 -promoter catalysts for grafting a second alpha-olefin onto a C.sub.6 -C.sub.12 alpha-olefin dimer to form a low volatility lubricating oil is described. The promoters include glycol ethers such as ethylene glycol monomethyl ether and propylene glycol monoethyl ether, and diisobutyl ether.
U.S. Pat. No. 4,218,330 to Shubkin describes dimerization of C.sub.12 -C.sub.18 alpha-olefin monomer with a BF.sub.3 -water complex and an excess of BF.sub.3. Unreacted monomer is distilled from the reaction product leaving mainly dimer with minor amounts of trimer and higher oligomers. The product is hydrogenated for use as a lubricant.
U.S. Pat. No. 4,436,947 to Morganson et al. discloses oligomerization of C.sub.6 -C.sub.20 olefins, such as 1-decene, with BF.sub.3 and a mixture of an aliphatic alcohol, an aliphatic ketone, and a polyol. The product is mainly trimer.
U.S. Pat. No. 4,982,026 to Karn describes polymerization of C.sub.2 -C.sub.6 alkene monomers with BF.sub.3 and a strong acid, such as phosphoric acid to produce a polymer having a molecular weight of from 250 to 500 and having a high vinylidene content.
U.S. Pat. No. 5,068,487 describes a process for producing products containing predominately dimers and trimers of alpha-olefins using a BF.sub.3 catalyst promoted by an alcohol alkoxylate.
U.S. Pat. No. 5,191,140 discloses a process for making alpha-olefin oligomers by use of BF.sub.3 promoted by at least two of water, alcohols and anhydrides to peak the reaction at lower molecular weight product.
In U.S. Pat. No. 5,396,013 it is shown that polyethers will moderate promoted BF.sub.3 -catalyzed oligomerizations to provide either predominately dimer- or trimer-containing oligomers.
U.S. Pat. No. 5,420,373 discloses a process for producing predominately dimer and trimer from C.sub.6 -C.sub.20 olefins, such as 1-decene, with BF.sub.3 and a hydroxy carbonyl promoter--i.e., a hydroxy ketone or a hydroxy aldehyde. Secondary promoters may also be used, namely aldehydes, alcohols, alcohol alkoxylates, carboxylic acids, ethers, ketones, and their mixtures.
The particular application for which the oligomer oils are used depends largely upon their viscosity, with viscosities of about 2-10 cSt at 100.degree. C. being preferred for general lubricating oil applications. These materials are, in general, mixtures of different percentages of dimer, trimer, tetramer, pentamer and, in the case of the higher viscosity products in this range, higher oligomers as well. To increase viscosity, processes are used which either produce more of the higher oligomers or some of the lower oligomers are removed such as by distillation.
Most lower viscosity dimer products are obtained as by-products of the production of higher viscosity synthetic oils. Because of increasing use of dimers in applications such as low temperature lubricants and drilling fluids, methods for their preferential production are of particular interest. Although higher oligomerization temperatures tend to increase dimer formation, use of such higher temperatures can cause corrosion of process equipment.